The present invention relates to a traveling water screen for screening and removing debris and fish from water, and more particularly to an improved basket for a traveling water screen which enhances fish survival.
Traveling water screens have been widely used in applications in which it is desired to screen debris and fish from large volumes of water such as a power plant which requires a large volume of cooling water. Water for this purpose is usually taken from a river or lake through an inlet water channel. Debris and fish enter the channel with the water flowing into channel and must be screened out of the water to prevent debris from clogging the condenser tubes and to prevent the fish from being killed by heat and impingement on the condenser tubes and other parts of the cooling system.
U.S. Pat. Nos. 5,242,583; 4,935,131; 4,582,601; 4,541,930; 4,360,426; 3,868,324 and 3,850,804 illustrate traveling water screens and are exemplary of the state of prior art relating to this field. As generally taught by this prior art, a traveling water screen typically includes an upstanding frame having a pair of spaced vertical support members. The support members are mounted adjacent the opposed sides of the water inlet channel, such that water flowing through the channel cannot flow around the sides of the water screen. As set forth more specifically in U.S. Pat. No. 5,501,793, the frame includes a boot portion adjacent the bottom of the channel and a head portion anchored in a horizontal shelf above the water in the channel. A foot shaft is supported for rotation at the boot portion. A head shaft is supported for rotation at the head portion of the frame, and a drive motor is connected to the head shaft. A pair of endless chains are positioned around sprockets mounted on the head and foot shafts. A plurality of screen baskets are mounted on the chains and arranged in a continuous train for movement with the chains about a circuitous vertical path defined around the head and foot shafts. Adjacent baskets are mounted in close edge-to-edge relationship, and small gaps are provided between the baskets to provide clearance for the baskets to travel around the head and foot shafts. On the upstream side of the frame, the train of baskets moves upwardly from the foot shaft toward the head shaft, thereby forming a substantially continuous upwardly moving screen on the upstream side of the frame. The gaps between adjacent baskets permit unscreened water to flow through the traveling water screen.
Each basket includes spaced apart upper and lower rail beams, a pair of spaced apart end plates at opposite ends of the rail beams, and a water screen extending between the end plates and the upper and lower rail beams. The lower rail beam can include an elongated, upwardly opening channel upstream of the water screen for recovering fish trapped against the screen. The fish are captured in the channel as the basket moves upwardly on the upstream side of the frame. When the basket moves around the head portion of the frame, the fish and water are discharged out of the channel and into a trough in the shelf for return to the river or lake away from the water intake channel. A portion of the recovered fish are killed or injured during captivity in the channel before being deposited into the return trough.
Most prior art basket designs inherently have a hydraulically violent or turbulent region inside the lower channel rail which contain the fish until they are eventually released from the traveling water screen. This violent or turbulent region leads to a high percentage of fatalities of all fish which are caught and handled by these prior art traveling water screens.
In an effort to overcome this prior art problem, and increase the recovery of live and uninjured fish, U.S. Pat. No. 5,501,793 provides a basket of specific geometry which creates a quiescent region in the lower channel rail of the basket.
The basket design of the '793 patent, although accomplishing its objective of creating a quiescent zone, does so at a relatively high cost because of the complex structural geometry of the bottom channel rail of the basket. In addition, because the channel rail is an integral part design of the basket of the '793 patent, the quiescent zone structure cannot be readily retrofitted on existing prior art baskets to provide the advantages of the quiescent zone.